Etching Gas

ABSTRACT

Disclosed is an etching gas provided containing CHF 2 COF. The etching gas may contain, as an additive, at least one kind of gas selected from O 2 , O 3 , CO, CO 2 , F 2 , NF 3 , Cl 2 , Br 2 , I 2 , XF n  (In this formula, X represents Cl, I or Br. n represents an integer satisfying 1≦n≦7.), CH 4 , CH 3 F, CH 2 F 2 , CHF 3 , N 2 , He, Ar, Ne, Kr and the like, from CH 4 , C 2 H 2 ,C 2 H 4 ,C 2 H 6 , C 3 H 4 , C 3 H 6 , C 3 H 5 , HI, HBr, HCl, CO, NO, NH 3 , H 2  and the like, or from CH 4 , CH 3 F, CH 2 F 2  and CHF 3 . This etching gas is not only excellent in etching performances such as the selection ratio to a resist and the patterning profile but also easily available and does not substantially by-produce CF 4  that places a burden on the environment.

TECHNICAL FIELD

The present invention relates to an etching gas used for producing thin film devices represented by IC, LSI, TFT and the like, and particularly to an etching gas that accomplishes both environmental performances and micro-patterning performances.

BACKGROUND OF THE INVENTION

In processes for producing semiconductor thin film devices, optical devices, super steel materials and the like, there have been produced various thin films, thick films and the like by means of CVD method, sputtering method, sol-gel method, vapor deposition method and the like. Moreover, in order to form a circuit pattern, gas etching for partially removing a thin film material has been conducted on semiconductors or in fabrication of semiconductors for IC, LSI, TFT and the like.

Hitherto, perfluorocarbons (PFCs) such as CF₄, C₂F₆, C₃F₈ and the like have been used as an etching gas in etching for forming circuits, in fabrication of thin film devices. However, these gases exist in the environment stably for a long period of time and therefore regarded as having high global warming potentials, so that their adverse influence on the environment has come to an issue.

For example, their GWPs known from the IPCC Fourth Assessment Report are as follows (on a 100 year scale):

CF₄: 7390

C₂F₆: 12200

C₃F₈: 8830

An etching gas partially having the structure of CF₃ group e.g. C₂F₆, C₃F₈ and the like generates active species exemplified by CF₃ radicals, ions and the like in a deposition room (a chamber) thereby exhibiting the etching effect; however, CF₃ active species are brought into contact with fluorine radicals or with fluorine active species of ions to be recombined thereto, thereby forming CF4 as a by-product.

Guidelines on the destruction of PFCs issued by Office of Fluorocarbons Control Policy, Global Environmental Issues Division of the Global Environment Bureau of the Ministry of the Environment (issued in March 2009) states that CF4 is the most undecomposable PFC in the environment and therefore it may not be sufficiently destructed under the destructing conditions similar to those for other fluorocarbons.

As a fluorine-containing etching gas having low global warming potentials and substitutable for PFC, there have been proposed COF₂, CHF₂OF (Patent Publication 1), CF₃COF (Patent Publications 2 and 3) and the like. These publications state it is possible to reduce by⁻production of CF₄, for example, by optimizing an etching condition for CF₃COF.

REFERENCES ABOUT PRIOR ART Patent Publication

Patent Publication 1: Japanese Patent Application Publication No. 2000-63826

Patent Publication 2: Japanese Patent Application Publication No. 2000-265275

Patent Publication 3: Japanese Patent Application Publication No. 2002-158181

SUMMARY OF THE INVENTION

As mentioned above, Patent Publications 1 and 2 state that by-production of CF₄ can be reduced by optimizing an etching condition for CF₃COF. However, it is considered difficult to fundamentally avoid a recombination of the CF₃ active species and the fluorine active species so long as the etching gas partially having the structure of CF₃ group is used. In view of the above, the optimized etching condition is found not to be optimized in respect of the micro-patterning speed and the patterning accuracy, which means that the aimed patterning accuracy and the like are restricted by the rate of CF₄ by-production. In fact, there are not a few cases difficult to constantly reduce by-production of CF₄ under a condition satisfying required performances such as the etching rate, the anisotropy, the aspect ratio, the resist ratio and the like.

In order to obtain a good anisotropy in etching that requires micro-patterning performances, a compound having a ratio of fluorine number to carbon number (F/C) close to 1 in a is demanded. For example, in the case of perfluorocarbons, F/C of CF₄ is 4, F/C of C₂F₆ is 3 and F/C of C₃F₈ is 2.7. As the carbon number gets increased, F/C thus approaches 1 or the above-mentioned requirement, but the boiling point also gets increased so as to become difficult to handle as gas. Moreover, F/C of CF₃COF is 2 and therefore it is not a satisfying value too.

In view of the above, an object of the present invention is to provide a novel etching gas which is not only excellent in etching performances but also easily available and does not substantially by-produce CF₄ that places a burden on the environment.

The present inventors had eagerly made studies on the above-mentioned object and thereby found that CHF₂COF can accomplish both the environmental performances and the environmental safety, with which the present invention has come to completion. More specifically, the present invention is as follows.

[Invention 1]

An etching gas used for etching semiconductors, dielectric substances or thin films formed of metals, comprising CHF₂COF.

[Invention 2]

An etching gas of Invention 1, wherein the semiconductors or the dielectric substances are a silicon-containing substance.

[Invention 3]

An etching gas of Invention 1, wherein the etching gas contains at least one kind of gas selected from O₂, O₃, CO, CO₂, F₂, NF₃, Cl₂, Br₂, I₂, XF_(n) (In this formula, X represents Cl, I or Br. n represents an integer satisfying 1≦n≦7.), CH₄, CH₃F, CH₂F₂, CHF₃, N₂, He, Ar, Ne and Kr, as an additive.

[Invention 4]

An etching gas of Invention 1, wherein the etching gas contains at least one kind of gas selected from CH₄, C₂H₂, C₂H₄, C₂H₆, C₃H₄, C₃H₆, C₃H₈, HI, HBr, HCl, CO, NO, NH₃, H₂, N₂, He, Ar, Ne and Kr, as an additive.

[Invention 5]

An etching gas of Invention 1, wherein the etching gas contains at least one kind of gas selected from CH₄, CH₃F, CH₂F₂, CHF₃, as an additive.

[Invention 6]

A method for etching semiconductor films, dielectric films or metal films, comprising the step of: using an etching gas of Invention 1.

[Invention 7]

A method for etching, of Invention 6, further comprising the step of: thereafter ashing by F₂ or O₂.

BRIEF EXPLANATION OF THE DRAWINGS

[FIG. 1A] A schematic cross section of a sample for etching used in Examples and Comparative Examples.

[FIG. 1B] A schematic cross section of a sample after etching (showing a case of having facets).

[FIG. 2] A schematic cross section of a remote plasma apparatus used in Examples and Comparative Examples.

DETAILED DESCRIPTION

The etching gas according to the present invention is not only characterized by placing a slight burden on the environment by virtue of its containing CHF₂COF but also exhibits good etching performances in semiconductor film-forming process, i.e., a performance of high etching rates, a performance of not bringing corrosion to the apparatus and the like. Hence the etching gas is useful for micro-patterning conducted on thin films by etching in semiconductor film-forming process.

The present invention will be hereinafter discussed in detail.

CHF₂COF can be readily and rationally synthesized by catalytic cracking of 1-alkoxy-1,1,2,2-tetrafluoroethane represented by CHF₂CF₂OR (where R is an alkyl group including Me, Et, n-Pr, iso-Pr, n-Bu, sec-Bu, iso-Bu, tert-Bu and the like) and used as a cleaning agent, a foaming agent or the like such as HFE-254pc (CHF₂CF₂OMe), HFE-374pc-f (CHF₂CF₂OEt) and the like. Moreover, HFE-254pc and HFE-374pc-f can be synthesized by adding methanol or ethanol to an industrially mass-produced tetrafluoroethylene so as to be greatly available compounds.

CHF₂COF has a boiling point of 0° C. and therefore serves as a highly convenient etching gas that can be handled as either liquid or gas. Additionally, CHF2COF is reacted with water to be decomposed into difluoroacetic acid (CHF₂COOH) and hydrogen fluoride (HF), so that usually its hazard can be eliminated by using a water scrubber. It is also preferable to use an alkaline water scrubber. Even in the event of passing the hazard-eliminating step so as to be emitted into the air, CHF₂COF is reacted with rain and steam in the air thereby being readily decomposed. Thus its environmental impact is extremely minimal.

As a point where CHF₂COF of the present invention is significantly different from the existing CF₃COF in property, it is possible to cite an easiness to establish a ketene structure. CHF₂COF is known to be able to take on a ketene structure represented by CF₂═C═O as shown in the following equation. In the case of CF₃COF, a reaction for taking on the ketene structure is an endothermic reaction calculated at 165.9 kcal. In order to develop this reaction a further activation energy is required in addition to the above free energy, so that the likelihood of this reaction can be said to be actually remarkably little.

CHF₂COF→CF₂═C═O+HF+48.9 kcal/mol

CF₃COF→CF₂═C═O+F₂+165.9 kcal/mol

The heat of reaction is a value calculated by B3LYP/6-311G+**.

As will be discussed in Examples, in the cases of using CHF₂COF as the etching gas, CF₄ was not detected at all even under variously modified conditions. It can be supposed from this fact that etching was developed through a vastly different mechanism from CF₃COF.

Furthermore, in the case of using CF₃COF, once generated CF₃ active species are brought into contact with fluorine active species with a certain probability to cause recombination thereby by-producing CF₄ (in an etching process employing plasma, for example). On the contrary, in the case of using CHF₂COF, by-production remains at CHF₃ which is relatively reasonably decomposable even if CHF₂ active species and fluorine active species are brought into contact with each other. Stochastically there is the possibility that CHF₃ is so decomposed as to form CF₃ active species and it is bonded to the fluorine active species again thereby to by-produce CF₄; however, it is easily supposed that such a probability is extremely small as compared to etching gases partially having the structure of CF₃ group (CF₃COF, etc.). For the above reasons CHF₂COF is considered not to substantially by-produce CF₄. As a matter of fact, by-production of CF₄ was not recognized in any of the Examples.

The etching gas containing CHF₂COF according to the present invention can be particularly preferably used for etching of; semiconductors when manufacturing semiconductor devices; dielectric substances; or thin films formed of metals.

As substances able to be etched by the etching gas containing CHF₂COF according to the present invention, it is possible to cite B, P, W, Si, Ti, V, Nb, Ta, Se, Te, Mo, Re, Os, Ru, Ir, Sb, Ge, Au, Ag, As, Cr, Hf, Zr, Ni, Co and their compounds deposited on semiconductor substrates such as silicon wafers, GaAs wafers and the like, metal substrates such as W, Ta, Mo and the like, insulating or dielectric substrates such as SiO₂, Al₂O₃, Ta₂O₃ and the like, glasses such as soda-lime glasses, borosilicate glasses and the like, or substrates formed of single crystals or polycrystals of other compounds or the like. Among them, this etching gas is particularly effective at etching oxides, nitrides, carbides, or composites of them. It is particularly preferable to use W, WSi_(x), Ti, TiN, Ta₂O₅, Mo, Re, Ge, Si₃N₄, Si, SiO₂ or the like, more preferably silicon-containing substances such as WSi_(x), Si₃N₄, Si, SiO₂ and the like, and much more preferably Si or SiO₂. The above-mentioned substances may be any of single crystal, polycrystal and amorphous form.

The etching gas of the present invention can be used for etching exemplified by RIE (reactive-ion etching), ECR (electron cyclotron resonance) plasma etching, microwave etching and the like, but not limited to these. Additionally, these kinds of etching processes are a common knowledge among the skilled in the art and can be referred to from publications as needed. The reaction conditions are not required particularly. When CHF₂COF is used, fluorine radicals reach recessed portions of channels and then CFy ions (where y represents an integer of from 1 to 3) enter there, with which etching develops in a longitudinal direction. Side walls are protected by deposition of a fluorocarbon polymer, thereby preventing an isotropic etching due to fluorine radicals and allowing an anisotropic etching. Moreover, since CHF₂COF contains oxygen (O), there is an advantage that the anisotropic etching can be developed while efficiently removing fluorocarbon films deposited on the side walls.

As a reason for the specifically good micro-patterning of CHF₂COF, it is possible to cite not only the fact that a ratio represented by F/C is 1.5 in the case of CHF₂COF while it is 2 in the case of CF₃COF but also the effect of the above-mentioned ketene which is polymerized to protect the side walls. It is also possible to remove organic substances including polymers, in such a manner as to carry out heating by using an oxidizing gas such as F₂, O₂ and the like or to carry out plasma ashing after conducting etching by using the etching gas of the present invention,

An etching method of the present invention is practicable under various dry etching conditions and allows the addition of various additives depending on the property, productivity, micro-patterning accuracy and the like of the target film. Inert gases exemplified by N₂, He, Ar, Ne, Kr and the like are usable as a dilutent, and more particularly, Ar is effective at stabilizing plasma and therefore provides an enhanced etching rate by a synergistic effect with CHF₂COF. Though the amount of the added inert gases depends on the configuration and performances of the apparatus such as the output, the amount of discharged gas and the like or on the properties of the target film, it is preferably 1/10 to 30 times the amount of flow of CHF₂COF.

With the addition of the oxidizing gas to CHF₂COF, it becomes possible to increase the etching rate and to enhance the productivity. Concrete examples thereof are O₂, O₃, CO₂, F₂, NF₃, Cl₂, Br₂, I₂, XF_(n) (In this formula, X represents Cl, I or Br. n represents an integer satisfying 1≦n≦7. Concrete examples are ClF, ClF₃, BrF, BrF₃, IF₅ and IF₇.). Though the amount of the added oxidizing gas depends on the configuration and performances of the apparatus such as the output and the like or on the properties of the target film, it is usually 1/20 to 30 times the amount of flow of CHF₂COF, more preferably 1/10 to 10 times the amount of flow of CHF₂COF. The addition exceeding 30 times impairs the excellent anisotropic etching performance of CHF₂COF and therefore not preferable. In the case of less than 1/20, the effect of the addition of the oxidizing gas cannot be sufficiently exhibited and therefore not preferable. In particular, the addition of oxygen allows selectively accelerating the etching rate on metals, and more specifically, greatly improves the selection ratio of the etching rate on metals thereby allowing a selective etching on metals. It is a matter of course that the addition of inert gases such as N₂, He, Ar, Ne, Kr and the like in addition to the oxidizing gas is acceptable as desired.

If reduction of the amount of fluorine radicals that accelerates the isotropic etching is required, the addition of a reducing gas exemplified by CH₄, C₂H₂, C₂H₄, C₂H₆, C₃H₄, C₃H₆, C₃H₈, HI, HBr, HCl, CO, NO, NH₃ and H₂ provides a good effect. The amount of the addition is preferably not larger than 10 times. When the addition exceeds the above, fluorine radicals that work on etching is significantly reduced thereby decreasing the productivity. Particularly, the addition of H₂ or C₂H₂ decreases the etching rate on Si while the etching rate on SiO₂ is not changed, so that the selectivity is enhanced. Thus it becomes possible to etch SiO₂ selectively against a foundation silicon.

Furthermore, gases having a carbon number of 1 such as CH₄, CH₃F, CH₂F₂ and CHF₃ are effective in a fine-tuning of the ratio of fluorine to carbon in the etching gas. The amount of the addition thereof also is preferably not larger than 10 times the amount of CHF₂COF. When the addition exceeds the above, the excellent etching performances of CHF₂COF get impaired. CO traps HF (which has been by-produced, for example when ketene is generated) in the form of HCOF and works as an etching agent in itself, so as to be efficiently used. The amount of CO to be added is from 10:1 to 1:5, preferably from 5:1 to 1:1 in a mole ratio represented by CHF₂COF:CO.

The pressure in the case of using the etching gas of the present invention is preferably not higher than 660 Pa (5 Torr) in order to perform anisotropic etching; however, pressures of not higher than 0.13 Pa (0.001 Torr) reduce the etching rate and therefore not preferable. The flow rate of gas to be used depends on the volume of a reactor of the etching apparatus and on the size of the wafer, but it is preferable to carry out etching at a flow rate of between 10 to 10000 SCCM. Moreover, the temperature for etching is preferably not greater than 400° C. High temperatures exceeding 400° C. are not preferable since etching tends to develop isotropically so that a desired patterning accuracy cannot be obtained and since a resist is excessively etched.

EXAMPLES

The present invention will be more readily understood with reference to the following Examples.

Examples 1 to 3 and Comparative Examples 1 and 2

There are shown examples where etching was conducted on an interlaminar insulating film (SiO₂) by using an etching gas of the present invention for contact hole-fabrication.

As a sample, a SiO₂ interlaminar insulating film 22 was formed on a single crystal silicon wafer 21. The sample was put to use upon forming on the SiO₂ film a resist mask 23 having apertures to serve as an etching mask. A sample before etching is shown in FIG. 1A.

A schematic cross section of an apparatus used in the experiment is shown in FIG. 2. By using a high-frequency source 3 (13.56 MHz, 50 W), etching gases (difluoroacetyl fluoride (CHF₂COF), oxygen (O₂), argon (Ar)) having been supplied from a gas inlet at flow rates shown in Table 1 were excited in a sapphire tube 7 attached to the top of a reaction chamber 1 thereby generating active species. The active species were supplied into the chamber by the flow of gas, upon which etching was conducted on a sample 12 fixed by a sample holder 11. Among the gas specimens, CHF₂COF, CF₃COF and CF₄ were introduced from a first gas inlet while O₂ was introduced from a second gas inlet, through a mass flow controller (though not shown).

The temperature of the substrate (or the sample holder 11) was set at 25° C., the pressure was set at 2.67 Pa (0.02 Torr), and the RF power density was set at 2.2 W/cm². A discharged gas was diluted with nitrogen supplied at 5 L/min on a discharge side of a mechanical booster pump, and then the concentration of CF₄ was quantified by calibration curve method with the use of FT-IR. Results of the above are shown in Table 1. Incidentally, “ND” shown in the Table refers to less than the floor limit for detection (0.05 volume %). The etching rate (Å/min) was determined in such a manner as to divide film thicknesses obtained before and after etching by an etching time. The film thicknesses were measured by an optical interferotype film-thickness meter. A sample after etching (in the case of having facets) is shown in FIG. 1B.

TABLE 1 Flow Flow CH₄ Rate Rate Etching Selection Concentration of Gas 1 of Gas 2 Rate Ratio to Aspect Patterning in Discharged Gas 1 SCCM Gas 2 SCCM Å/min Resist Ratio Profile Gas Example 1 CHF₂COF 50 None — 4058 6 6 or No facet, Less than more Good side floor limit for wall detection Example 2 CHF₂COF 10 Ar 200 4121 6 6 or No facet, Less than more Good side floor limit for wall detection Example 3 CHF₂COF 50 O₂  10 15598 7 6 or No facet, Less than more Good side floor limit for wall detection Comparative CF₃COF 50 None — 3218 5 6 or Partially 0.11% Example 1 more having facets and gouges in side wall Comparative CF₄ 50 None — 608 4 5 Partially 0.18% Example 2 having facets and gouges in side wall CHF₂COF: Difluoroacetyl fluoride CF₃COF: Trifluoroacetyl fluoride O₂: Oxygen Ar: Argon CF₄: Carbon tetrafluoride

EXPLANATION OF REFERENCE NUMERALS

1 Chamber

2 Earth

3 High-frequency source

4 First gas inlet

5 Second gas inlet

6 Third gas inlet

7 Sapphire tube

8 Induction coil

9 Electronic pressure meter

10 Discharged-gas line

11 Sample holder

12 Sample

21 Silicon wafer

22 SiO₂ interlaminar insulating film

23 Resist mask

24 Facets 

1. An etching gas used for etching semiconductors, dielectric substances or thin films formed of metals, comprising: CHF₂COF.
 2. An etching gas as claimed in claim 1, wherein the semiconductors or the dielectric substances are a silicon-containing substance.
 3. An etching gas as claimed in claim 1, wherein the etching gas contains at least one kind of gas selected from O₂, O₃, CO, CO₂, F₂, NF₃, Cl₂, Br₂, I₂, XF_(n) (In this formula, X represents Cl, I or Br. n represents an integer satisfying 1≦n≦7.), CH₄, CH₃F, CH₂F₂, CHF₃, N₂, He, Ar, Ne and Kr, as an additive.
 4. An etching gas as claimed in claim 1, wherein the etching gas contains at least one kind of gas selected from CH₄, C₂H₂, C₂H₄, C₂H₆, C₃H₄, C₃H₆, C₃H₈, HI, HBr, HCl, CO, NO, NH₃, H₂, N₂, He, Ar, Ne and Kr, as an additive.
 5. An etching gas as claimed in claim 1, wherein the etching gas contains at least one kind of gas selected from CH₄, CH₃F, CH₂F₂, CHF₃, as an additive.
 6. A method for etching semiconductor films, dielectric films or metal films, comprising the step of: using an etching gas as claimed in claim
 1. 7. A method for etching, as claimed in claim 6, further comprising the step of: thereafter ashing by F₂ or O₂. 